1. Field of the Invention
The present invention relates to a method of measuring the optical characteristic of an optical system, and an exposure apparatus having an arrangement suitable for the measurement method.
2. Description of the Related Art
In recent years, the tolerance of the overlay accuracy of an exposure apparatus has decreased to the degree that an overlay accuracy of 10 nm or less is required.
Under the circumstance, the measurement accuracy of the distortion aberration, of a projection optical system is also being required to be 10 nm or less. An example of a distortion aberration measurement method will be explained herein. First, a point source is set at a first position in the object plane of an optical system to be measured, and the position of an image of the point source is measured on the image plane side of the optical system. At this time, the position of the point source on the object side is acquired at once.
Next, a point source is set at a second position in the object plane of the optical system to be measured, and the position of an image of the point source on the image plane side and that of the point source on the object side are acquired in the same way. This operation is repeated a plurality of times to acquire the positions of a plurality of image points corresponding to those of a plurality of object points. Finally, calculating a misalignment between the position of an ideal image point and the measured position of each image point makes it possible to measure the distortion aberration. It is possible to measure the field curvature associated with the distortion of the image plane in the optical axis direction by the same method.
Japanese Patent Nos. 3352298 and 3728187 describe a method of calculating the distortion aberration and the field curvature by configuring a wavefront measurement interferometer using a Fizeau interferometer, and measuring the positions of object and image points using the position coordinates of the TS focus and the center of curvature of the RS, and a value calculated from the tilt and spherical component of the wavefront.
To measure the distortion aberration and the field curvature mentioned above, it is necessary to precisely measure the positions of object points in the optical system to be measured, and the positional relationship between the object and image points. A position measurement error at each measurement point is a misalignment of the image point with respect to the object point, which appears as the distortion aberration and the field curvature, resulting in deterioration in the measurement accuracy. Position measurement errors may occur in stage position measurement systems which measure the positions of an object-side stage for moving the object point, and an image-side stage for moving an image point measurement device (e.g., the RS of the Fizeau interferometer). Detailed causes for the position measurement errors include, e.g., orthogonality errors of the measurement systems, a shape error of a reference mirror when a laser interferometer is used, and drifts and fluctuations in the surrounding environmental factors such as the temperature.